Original Date: 03/17/1997
Revision Date: 01/18/2007
Best Practice : Free Cooling with Evaporative Fill Media Pads
Approximately 15 years ago, Polaroid’s facilities engineers began retrofitting and installing new process, manufacturing, and office space heating, ventilating, and air conditioning (HVAC) systems using evaporative fill media pads. As a result, Polaroid achieved virtually free humidification and cooling year round while maintaining a 52°F dew point discharge temperature. Concurrently, the air handling units were redesigned and retrofitted to incorporate the advantages of a positive pressure, blow-through system. This modification reduced the possibility of chilled water coils freezing in the winter due to air stratification; eliminated the need for a preheater coil; and extended the life expectancy of equipment by housing the fan, motor, and drive in a dry, contaminant-free environment.
Before the retrofit, Polaroid’s HVAC systems were installed with a draw-through design (Figure 2-9) in which the outside air and the return air were drawn into a plenum where the prefilter, preheat coil, chilled water coil, and steam humidifier were before the fan. This design allowed air to return from the conditioned space and mix with the minimum-required outside air make-up. As a result, portions of the chilled water coils became susceptible to freezing damage caused by the improper mixing of the cold outside air, warm return air, and warm air from the preheat coils (installed primarily to preheat the air and protect the chilled water coil). As the cold air and the warm air in the plenum stratified, pockets of cold air would pass over the surface of the chilled water coil, freeze on contact, and rupture the copper lines in the coil. This event not only shut down the air conditioning system for comfort cooling, but in most cases, shut down the process cooling and the production line in that area. The design is also inherently inefficient. On cold days, the air in the plenum is often preheated, and then cooled to a temperature required to condition the space, typically 55°F to 60°F. In addition, humidity requirements were satisfied by a steam humidifier located before the fan, motor, and drive which subjected internal parts to moisture and encouraged rust, dirt, bacteria formation, and long-term destruction of the fiberglass insulation in the plenum housing.
By retrofitting the units and incorporating the use of a 16-inch evaporative fill media pad, Polaroid eliminated the original design problems and provided virtually free cooling to the conditioned space. In the new blow-through design, the fan was placed before a high-efficiency filter, the chilled water coil, and the evaporative fill media. This design allows the stratified air to mix properly within the fan before reaching the coils which eliminates preheat coils and potential freezing hazards. By using evaporative fill media pads, the design allows the unit to be constantly saturated with recirculated and filtered water and provides the required cooling and humidity conditions to satisfy the controlling dew point of 52°F.
Benefits of Polaroid’s new design are multiple and varied. Energy is saved through evaporative cooling by using the evaporative fill and a two-stage filtration, and eliminating the preheat coil. Other benefits include eliminating cost humidification; reducing the potential of the chilled water coil freezing in winter by allowing the stratified air to mix properly in the plenum housing; eliminating contamination into the unit via air leakage by placing the air under a positive pressure after the final filters; and housing the fan, motor, and drive in a dry environment by eliminating the direct steam injection into the air stream for humidification.
Figure 2-9. Heating, Ventilating, and Air Conditioning Systems
For more information see the
Point of Contact for this survey.